Squarebody Suburbans are known for the faulty back glass tailgate motors, wiring, and difficulty. A few years ago I installed a relay system to positively supply the motor with uninterupted power, while triggering the direction via the relay. Really only took a few hours on a saturday afternoon, to build a bracket, clean up the wiring and install the relays. This same concept is what LMC sells in a door window kit, so their kit could also be retro fitted as well. I used the relay wiring blocks from NAPA with the AR272 relays.
Good luck and hope this helps you.
BTW, we are removing the tailgate in the fall and swapping on barn doors...